1. Field of the Invention
The present invention is related to the field of oil exploration. In particular, the present invention is related to a method and apparatus for a down-hole antenna.
2. Description of the Related Art
Logging tools are used in boreholes drilled into the earth""s formation to search for fluids and other minerals. In measurement while drilling (MWD) operations, measurements of the earth""s formation are obtained while the borehole is being drilled. Simultaneously with the drilling of the borehole, measurements of the earth""s formation are obtained at several distances along the axis of the borehole. Typically, a sensor used to measure characteristics of the earth""s formation comprises a cylindrical body (housing) that forms a section of a drillstring. In the case of a sensor that measures resistivity of the earth""s formation (i.e., a resistivity probe), the sensor has at least one transmitting antenna and one or more receiving antennas disposed along the length of the housing. The transmitting antenna generates electromagnetic waves that propagate through the earth""s formation in the region surrounding the borehole, and the receiving antenna detects the waves as they pass the respective receiving antennas. By measuring the amplitude and the phase of the received waves the resistivity of the earth""s formation may be determined.
The antennas of the resistivity sensor comprise one or more coils of wire that are placed in grooves around the circumference of the housing and filled with non-conductive material to permit the transmitted waves to couple from the antenna to the earth""s formation. Each antenna is electrically connected by wires to corresponding transmit and receive circuitry. The transmit and receive circuitry may be placed either in cavities on the collar itself or on a probe that is coaxially located within the collar. In the case of the coaxial probe, the connection with the antenna is usually via electrical wires or spring loaded contacts that connect the probe to the antennas. However, in both cases during MWD operations as the collar is subject to severe stresses (e.g., shear, torsion, etc.) and as the collar is subject to wear and erosion, the antenna coils and the wires that connect the antenna coils to the corresponding transmit and receive circuitry are subject to fail.
A failure of a sensor during drilling operations is very expensive and time consuming. Not only does the sensor have to be pulled out of the borehole, but also, the sensor (weighing several hundred pounds) has to be shipped to a repair facility for rebuilding.